1. Field of the Invention
This invention relates to an automatic guided vehicle having three wheels: one front wheel and a pair of rear wheels, the body of the vehicle being capable of travelling by driving the wheels.
2. Discussion of the Prior Art
An automatic guided vehicle of this kind has been known, which travels along a track formed on a floor by a white line or aluminum foil while detecting the track by means of an optical track sensor. In this type of conventional vehicle having, for example, three wheels: one front wheel and a pair of rear wheels, the front wheel can be driven through a connection to a driving system and can be automatically steered by a steering system while the rear wheels are freely rotatable but incapable of being driven by any driving system being steered.
Another known example of the conventional automatic guided vehicle is a four-wheel vehicle having front and rear wheels and on the opposite sides of a center axis having left and right wheels each of which is provided with a driving motor. In this example, all wheels are set in the forward direction and the left and right wheels are driven by the driving motors so as to perform the ordinary travelling operation of the vehile. The vehicle moves straight by equalizing the speeds of rotation of the left and right wheels and turns left or right by changing the speed ratio therebetween. At least the direction of the front wheels among the front and rear wheels can be freely changed so as to follow the motion caused when the left and right wheels are steered by changing the speed ratio therebetween. If the front and rear wheels are rotated in the reverse directions at the same rotational speed while the front and rear wheels are being fixed such as to be perpendicular to the direction of the front and rear of the vehicle body, the body of the vehicle can be turned about the point of intersection of the lines which connect the front wheel to the rear wheel and the left wheel to the right wheel.
However, the former type of conventional vehicle having three wheels is designed to travel in the forward direction alone while a track sensor provided on the front wheel assembly is detecting the track, so that the vehicle cannot turn and move off the track. It is therefore necessary for the minimum radius of curvature of the track to be large enough to enable the front wheel to be steered along the track. In order to enable the vehicle to turn, it is necessary for the vehicle to go off the track and be manually turned, or to provide a looped track having the minimum radius of curvature. A large additional space is thereby required. Moreover, when there are provided a pair of parallel tracks, these tracks must be connected to each other to ensure that the vehicle can move from one of these tracks to the other.
In the case of the latter type of vehicle having four wheels, it is necessary to synchronize left and right driving motors with each other or to adjust the speed ratios of these motors in relation to the steering operation. While this type of vehicle is capable of turning on the track, it can turn only about the point of intersection of the four wheels. Also it is not capable of laterally moving off the track.